1. Field of the Invention:
The present invention relates to a process for chemical treatment of materials prior to vitrification. In particular, the present invention relates to a process for treating alkaline waste materials such as radioactive wastes, hazardous chemical wastes, and mixed radioactive,. and hazardous chemical wastes to produce a redox-balanced feed to a vitrification melter, and to a waste glass composition made by the process.
2. Discussion of Background:
Many industrial processes generate hazardous wastes in the form of aqueous. waste streams, sludges and slurries, aqueous supernate, incinerator ash, incinerator off gas condensate, and so forth. As used herein, the term "hazardous waste" means wastes containing substances commonly recognized as hazardous, including but not limited to chemical wastes, high level radioactive wastes, mixed chemical and radioactive; wastes, heavy-metal-containing wastes, and organic chemicals. Hazardous wastes must be treated and stabilized before disposal, for example, by encapsulation in a stable, durable product for long-term storage in an approved facility. Glass is stable and extremely durable, therefore, it is an environmentally acceptable waste form for hazardous wastes, especially radioactive wastes.
Processes for the recovery of actinide elements from spent nuclear fuel generate highly corrosive wastes that must be treated before mixing with glass formers ("frit") in order to ensure a stable, durable glass product. For example, Horwitz, et al. (U.S. Pat. No. 4,162,230) recover americium, curium and rare earths from a feed solution by contacting with nitric acid; neptunium and plutonium are recovered with a combination of nitric acid and formic acid. The aqueous waste solutions generated by the process are combined and solidified for long term storage. Sasaki, et al. (U.S. Pat. No. 5,190,623) lower the corrosiveness of metal ion-containing nitric acid solutions by placing a cathode in the metal ion-containing nitric acid solution and an anode in a nitric acid solution, with a membrane separating the two solutions. When a constant voltage or current is applied between the electrodes, high-valence metal ions (Ru(VIII), Ce(IV), Cr(VI), Fe(III)) in the nitric acid solution are reduced at the cathode to lower-valence, less corrosive states; nitrogen oxides generated by reduction of these high-valence ions provide a reducing atmosphere that prevents lower-valence ions (Ru(III or II), Ce(III), Cr(III), Fe(II)) from being oxidized to higher-valence states. Drobnik, et ,al. (U.S. Pat. No. 4,144,186) and Drobnik (U.S. Pat. No. 3,673,086) add formic acid to nitric acid-containing and/or nitrate-containing wastes that result from reprocessing of irradiated fuels. The formic acid destroys free nitric acid and any transition metal nitrates that are present in the wastes, reduces cations to lower valence states, and reduces noble metal ions to the metallic state. The denitrated wastes are spray-dried, calcinated, mixed with glass formers and vitrified.
FIG. 1 shows a typical waste treatment apparatus 20, where an alkaline waste stream 22 is input into a first vessel 24. Waste stream 22 may contain a variety of hazardous substances, as hereinabove defined. For example, waste stream 22 may result from a nuclear fuel reprocessing operation such as the Purex process, wherein spent fuel is dissolved in nitric acid, uranium and plutonium are recovered by solvent extraction, and various fission products are removed and processed as wastes. Afterwards, sodium hydroxide is added to the acidic waste for storage.
Alkaline wastes, especially wastes with pH greater than approximately 12, have high yield stress and consistency, and are hard to pump. To improve the rheology of waste stream 22, the material in stream 22 is neutralized by mixing it with acid supplied from an acid input stream 26. The acidified material may be transferred to an evaporator 28, where the solids concentration of waste 22 is adjusted by evaporating excess water. Alternatively, the solids concentration of waste 22 is adjusted in vessel 24. Elemental mercury contained in waste 22 is recovered by steam stripping in a second vessel 30. The acidified waste material is transferred to a third vessel 32, where it is mixed with a slurry 34 containing ground glass formers and adjusted to a solids content of no more than approximately 50 wt. % to produce a melter feed 36. Feed 36 is transferred to a melter 38, where it is processed by means well known in the art. Off-gas (CO.sub.2, NO, NO.sub.2, H.sub.2, etc.) generated by acid-base neutralization reactions is vented from evaporator 28, and condensate from vessels 30 and 32 is transferred to a condensate tank 40 for recovery and treatment.
Incoming waste stream 22 is alkaline, and, depending on the source, may contain alkali metal hydroxides, alkaline earth metal hydroxides, transition metal hydroxides, mercury (II) hydroxide, mercury (II) oxide, MnO.sub.2, oxides, carbonates, nitrites, nitrates, phosphates, sulfates, and small quantities of noble metals. Mercury is corrosive to the off-gas system of melter 38, and MnO.sub.2 in melter feed 36 causes foaming in melter 38. Therefore, waste 22 must be treated with both an acid and a reductant to produce an acceptable melter feed 36: an acid (supplied by stream 26) to lower the pH of the waste, and a reductant to chemically reduce any mercury to Hg for subsequent stream stripping, and reduce MnO.sub.2 in the waste.
Waste 22 may be treated by adding formic acid (HCOOH, CH.sub.2 O.sub.2) via input stream 26. Formic acid is unique in that it functions as both an acid and a chemical reductant or reducing agent: an acid to lower the pH of waste 22, and a reductant to destroy nitrites in the waste, reduce mercury compounds to elemental mercury for steam stripping in vessel 30, and reduce MnO.sub.2 to the Mn(II) (Mn.sup.++) ion to prevent foaming in melter 38. The amount of formic acid that is added to waste 22 depends on the composition of the waste, including the quantities of alkali metal hydroxides, alkaline earth metal hydroxides, carbonates, mercury compounds, MnO.sub.2 and nitrites present in the waste. Formic acid may be supplied via input stream 26, or as a constituent of the incoming waste stream.
Use of formic acid as an acidifying and reducing agent results in an acceptable feed for melter 38, however, hydrogen is generated during treatment of waste 22 when the waste contains noble metals such as Rut, Rh and Pd. Formic acid reduces noble metal compounds in waste 22 to metallic states, which then cause some of the remaining formic acid to decompose catalytically into H.sub.2 and CO.sub.2 as follows: ##STR1##
If only formic acid is used to treat waste 22, the nitrate concentration in melter feed 36 is often insufficient. The formate/nitrate balance is upset and feed 36 is too reducing. An overly reducing melt causes precipitation of metals and/or metal sulfides from feed 36 into melter 38, potentially shorting out the melter electrodes and thereby decreasing melter operating lifetime. In addition, hydrogen gas is generated, and suitable equipment is required to prevent a flammable atmosphere in the process and off-gas vessels.
There is a need for a process for preparing alkaline wastes for vitrification that produces less gaseous hydrogen than presently-used methods, while producing a redox-balanced melter feed that insures a durable vitrified product and proper melter operation. The process should acidify the wastes, reduce mercury compounds in the wastes to elemental mercury, and reduce MnO.sub.2 to the Mn(II) ion.